The nuclear receptors are a superfamily of ligand-activated transcription regulators that control many important cellular processes in humans. These soluble, intracellular proteins are modular constructs that contain a C-terminal ligand binding domain (LBD), a central DNA binding domain (DBD), and an N-terminal transcriptional transactivation domain (AF-1). The ligands that activate these receptors by binding to the LBD are lipophilic small-molecule hormones such as steroids, retinoids, and thyroid hormone. The nuclear receptor superfamily is arguably the most important class of biomedical targets presently known; drugs that target nuclear receptors are either currently used or under development for a wide range of indications including inflammation, hypertension, osteoporosis, hypercholestermia, diabetes, and various cancers. Important progress made over the past few years in structural and cell biology has led to a basic understanding of some of the bioregulatory mechanisms of nuclear receptors. The field is now at a critical point where the tools of organic chemistry can be used to reveal the detailed mechanistic underpinnings of how this important class of transcriptional regulatory proteins work. The research described in this proposal uses a chemistry-based approach to address two central questions concerning nuclear receptor transcriptional regulation: 1) ow do agonist ligands turn receptors on and how do antagonist ligands turn receptors off?; and 2) what are the molecular origins of the tissue-specific actions of nuclear receptor ligands? Three different nuclear receptors will be studied in the research plan. In Aim 1, the thyroid receptor will be used as a model to test a general structure-based hypothesis for nuclear receptor antagonist design. In Aim 2, the estrogen receptor will be used to examine a family of non-steroidal agonist and antagonist ligands structurally related to tamoxifen and reloxifene which are drugs that show tissue-specific estrogen receptor actions. In Aim 3, the receptor will be used in the development of non-steroidal ligands that are selective for specific glucocorticoid response elements. These ligand-based studies could provide key insight into the bioregulatory mechanisms of nuclear receptor function, and could provide new insight into the design of safer and more effective therapeutics.